1. Field of the Invention
The present invention relates to semiconductor wafer processing equipment and, more particularly, to a method and apparatus for exciting a plasma within a semiconductor wafer processing system using a complex radio-frequency (RF) waveform as a plasma excitation waveform.
2. Description of the Background Art
Traditionally, plasma-enhanced reactive ion semiconductor wafer processing systems contain an anode and cathode within a vacuum chamber. The cathode typically forms a pedestal for supporting a semiconductor wafer within the chamber and the anode is formed of the walls and/or top of the chamber. To process a wafer, a reactive gas is pumped into the vacuum chamber and the anode and cathode are driven by a single sinusoidal frequency RF source to excite the reactive gas into a plasma. The single frequency is typically 13.56 MHz, although frequencies from 100 kHz to 2.45 GHz are often used, with the occasional use of other frequencies. More specifically, a single frequency, sinusoidal RF signal is generally applied to the reactive gas within the chamber at a relatively high-power level, e.g., 3 kilowatts. The RF power excites the reactive gas, producing a plasma within the chamber proximate to the semiconductor wafer being processed. Such plasma-enhanced reactive ion processing has been used in etch and chemical vapor deposition processes.
More recently, multiple sinusoidal frequencies have been used to excite a plasma within a vacuum chamber. In such systems, a cathode/anode bias circuit is driven with a first RF frequency and an antenna or coil, proximate the vacuum chamber, is driven with a second RF frequency. Thus, each circuit is coupled to a separate and distinct RF power delivery system consisting of separate RF oscillators, preamplifiers, power amplifiers, transmission lines, and matching networks that supply independent sinusoidal RF frequencies at high power levels to each of the plasma excitation circuits. Such redundancy of oscillators and other circuitry is costly and complex.
Therefore, there is a need in the art for a method and apparatus for generating complex waveforms for plasma excitation within a process chamber, where the waveforms have multiple frequency components that are both harmonically and non-harmonically related.